Modelling the early time behaviour of type Ia supernovae: effects of the 56Ni distribution

Recent studies have demonstrated the diversity in type Ia supernovae (SNe Ia) at early times and highlighted a need for a better understanding of the explosion physics as manifested by observations soon after explosion. To this end, we present a Monte Carlo code designed to model the light curves of radioactively driven, hydrogen-free transients from explosion to approximately maximum light. In this initial study, we have used a parametrised description of the ejecta in SNe Ia, and performed a parameter study of the effects of the ⁵⁶Ni distribution on the observed colours and light curves for a fixed ⁵⁶Ni mass of 0.6 M_⊙. For a given density profile, we find that models with ⁵⁶Ni extending throughout the entirety of the ejecta are typically brighter and bluer shortly after explosion. Additionally, the shape of the density profile itself also plays an important role in determining the shape, rise time, and colours of observed light curves. We find that the multi-band light curves of at least one SNe Ia (SN 2009ig) are inconsistent with less extended ⁵⁶Ni distributions, but show good agreement with models that incorporate ⁵⁶Ni throughout the entire ejecta. We further demonstrate that comparisons with full UV OIR colour light curves are powerful tools in discriminating various ⁵⁶Ni distributions, and hence explosion models.